The gemara explains that the source for R. Yehuda that chometz must be burned rather than using other methods of destruction is a kal v'chomer from Nosar. The Rabbonon respond to R. Yehuda that if the source is going to be a kal v'chomer from no'sar (rather than a ma matzinu), chometz must be an absolute chumrah. However, here the chumra is a chumra that will lead to a kulah - לא מצא עצים לשרפו יהא יושב ובטל והתורה אמרה תשביתו שאור מבתיכם - If one doesn't have wood do burn chometz they will end up not burning it and be in violation of tashbisu.
Many Meforshim ask, that if one doesn't have wood to burn it they will be mevatel, thereby avoiding any problems. Why does the gemara assume that it will lead to a kulah since on a Torah level being mevatel the chometz is sufficient? Because of this question the kolbo (on of the rishonim) proves that chometz that one is aware of cannot be batul. Bitul is a special method that only works for chometz that one is unaware of. This approach can certainly not fit with Tosafos 21a (mentioned on this blog) who holds that for chometz that one is unaware of there is no violation of בל יראה ובל ימצא even without bitul. According to Tosafos biutl is only necessary for chometz that one is aware of, so it must be that bitul even works on chometz that one is aware of.
The Tzlach explains that according to Tosafos 12b who says that R. Yehuda demands burning only after the z'man issur, but prior to midday on erev pesach one can destroy chometz however they choose, there is no question. The Rabbonon are saying to R. Yehuda, after midday on erev pesach when one can no longer be mevatel (since it is already assur b'hana'ah) and you R. Yehuda insist on burning, it will lead to a leniency because one who doesn't have wood will not be able to fulfill tashbisu by destroying their chometz.
Another approach is that according to Tosafos 4b that bitul is not learned from tashbisu, rather it is a method of hefker, the Rabbonon may be saying to R. Yehuda that if one would not have firewood they could not perform the mitzvas aseh of tashbisu. They can certainly rid themselves of chometz using bitul or other methods of destruction, but they would not fulfill the mitzvah of tashbisu. However, according to Rashi 4b who says that Tashbisu is the source of bitul, and Rashi on 12b who says that after the z'man issur one can use any method of destruction it would seem that bitul is a perfectly legitimate way to fulfill tashbisu prior to the z'man issur and other methods are fine after the z'man issur, so what is the Rabbonon's question on R. Yehuda?
The Tzlach explains that Rashi on 4b who says that bitul is learned from tashbisu goes according to the Rabbonon that chometz can be destroyed in any method one chooses, therefore bitul can also be a fulfillment of tashbius. But, according to R. Yehuda who insisits on burning, bitul is not a fulfillment of tashbisu.